Motion during magnetic resonance imaging (“MRI”) scans is problematic, frequently prohibitively, in many types of studies. It often leads to artifacts, such as ghosting, which can severely degrade image quality. In studies where repeated scans at the same location need to be acquired, such as in functional MRI experiments, it is crucial that the scanning region within the subject be reliably maintained, requiring that motion be prevented or compensated for, otherwise incorrect interpretation of the results may occur. Further, in imaging of the flexing joints, it is also important to acquire images from the same anatomic location and orientation, which requires dynamic tracking of the scanned region.
In a prior motion correction technique, motion occurring within the scanned plane is corrected during the post-processing of the data. In another motion correction technique, data can be acquired in such a way that in-plane motion is compensated for. However, when through-plane motion takes place, images cannot be repaired retrospectively, because the data is erroneously obtained from an incorrect anatomic location in the subject. In addition, the use of restraining devices to immobilize patients for a significant period of time, to prevent erroneous data acquisition, has proven very uncomfortable.
In another prior motion correction technique, a magnetic resonance-based method for motion tracking is employed. In this method three liquid markers, each in its own transmit/receive coil, are placed on a subject's head during an MRI scan and the magnetic resonance frequencies for the samples are determined in the sequential presence of three magnetic field gradients that span Cartesian space. Because resonance frequency in the presence of a gradient can be correlated with a coordinate along the gradient direction, the positions for each marker can be determined and the orientation of the subject's head can be inferred therefrom. Thus, motion is traced by updating the markers' positions, followed by the determination of the rigid body transformation with respect to the initial coordinates.
In an alternative known motion correction technique, orbital navigator echoes with circular k-space trajectories can also be used to prospectively determine and adjust for translational motion (from the phase differences between the current echo and the reference echo) and rotational motion (from the shift in the magnitude profile of the current echo compared to the reference echo).
Another motion correction technique employs scanning plane adjustments to prospectively compensate for slow translational and rotational motion (position updating is performed every few seconds).
The aforementioned prospective motion correction techniques cannot be used in the background during the MRI scans. Rather, they can only be used in the time periods between MRI scans, because they rely on the use of magnetic field gradients and the same excitation frequency as the MRI scans. Therefore, motion that occurs during the MRI scans cannot be corrected, and additional time is required in between the scans for motion compensation steps.
Another known prior technique employs a laser position detection method to correct for head motion during MRI scans. In this method, motion is optically detected, and real-time feedback is possible by way of supplying six rigid-body motion parameters to a pulse program for compensatory gradient adjustments. This method can be run in the background of the MRI scans. However, a direct line of sight between three retroreflectors attached to a patient's head and the position-sensitive detectors outside of the magnet is necessary. In addition, the equipment required is complicated and expensive.
Thus, there is a need for a system and method capable of correcting for the motion of a subject during an MRI scan, independently of magnetic field gradients, which requires little additional hardware and is run in the background of an MRI scan.